At an interface between two transparent media of different refractive index (e.g., glass and water), light coming from the side of higher refractive index is partly reflected and partly refracted. Above a certain critical angle of incidence, no light is refracted across the interface, and total internal reflection is observed. While incident light is totally reflected the electromagnetic field component penetrates a short (tens of nanometers (nm)) distance into a medium of a lower refractive index creating an exponentially attenuating evanescent wave. If the interface between the media is coated with a thin layer of metal (e.g., gold), and light is monochromatic and p-polarized (i.e., polarized parallel with respect to a plane on which the light is incident), the intensity of the reflected light is sharply reduced at a specific incident angle (called surface plasmon resonance (SPR)) due to the resonance energy transfer between the evanescent wave and surface plasmons. The resonance conditions are influenced by the material adsorbed onto the thin metal film.
Surface plasmons, also termed “surface plasmon polaritons”, are surface electromagnetic waves that propagate in a direction parallel to a metal/dielectric (or metal/vacuum) interface. Since the wave is on the boundary of the metal and the dielectric, these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the metal surface. In one common configuration, termed the Kretschmann configuration, the thin metallic layer is disposed onto a transparent substrate (e.g., glass). Light illuminates the thin metallic layer through the transparent substrate, and an evanescent wave penetrates through the thin metallic layer. The plasmons are excited at the opposite side of the film thin metallic layer.
Surface plasmon resonance spectroscopy has been used as an analytical technique in various biological applications wherein a molecule is bound to a receptor attached to the metal layer. Surface plasmon resonance spectroscopy has also been used to detect organic vapors. For example, the use of MAKROLON M2400 polycarbonate from Bayer MaterialScience AG, Leverkusen, Germany) as a dielectric layer for use in SPR has been reported by Kieser et al. in Analytical Chemistry, 2002, vol. 74, pp. 4781-4787. MAKROLON polycarbonate is reported therein as being a microporous glassy polymer and having a mean size of the pores of 0.1 nm3.